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Samoan mothers have one of the lowest rates of postnatal depression in the world, researchers have discovered.

An Auckland University of Technology study has found an 8 per cent depression rate for Samoan mothers, half that of Pacific Island mothers in total.

The rate for all New Zealand mothers is 20 per cent.

Researchers hope that when they find out why Samoan women escape the baby blues, others can be helped to deal with them.

The findings come from the Pacific Islands Families First 2 Years of Life research project - a study of 1398 South Auckland infants - based at the university's National Institute for Public Health and Mental Health Research.

The dean of the faculty of health, Professor Max Abbott, said understanding why Samoan mothers had such low rates, even when exposed to serious risk factors, could be of great benefit to women worldwide.

Professor Abbott was aware of only two other studies that showed similarly low post-natal depression results - in Japan and the Netherlands.

He said it was too early to suggest reasons Samoan women had lower rates of postnatal depression than other Pacific Island women.

"We we will go back and do further interviews with the participants."

Professor Abbott said there was much to be learned about why Samoan mothers did so well, even when exposed to high levels of adversity.

"There may be protective factors operating that could benefit other mothers."

The study found 16.5 per cent of the Pacific Island mothers had experienced postnatal depression six weeks after childbirth, which was sightly lower than the 20 per cent rate of New Zealand mothers generally.

Professor Abbott cautioned that the average figure masked large differences between Pacific Island groups, which highlighted the importance of considering the differing ethnic needs in areas such as health delivery.

"One glove does not fit all."

He said postnatal depression was a significant health issue.

"While widespread, it is often undetected and untreated. It was once thought to be a culture-bound syndrome peculiar to urban Western societies, but research has found it is universal and that depression during pregnancy is also commonplace."

It had a serious impact on mothers' wellbeing and quality of life, and recent international research had found that postnatal depression led to disturbances in partner and mother-infant relationships and affected children's long-term emotional and intellectual development.